Typical wavelength division multiplex (WDM) networks comprise a number of nodes, which are interconnected and transmit and receive WDM signals. Typically, the nodes comprise reconfigurable optical add-drop multiplexers (ROADM) for adding additional signals on available channels to a WDM signal and/or for dropping WDM signals from particular channels of received WDM signals in the nodes. A channel designates a wavelength or wavelength range which is used for signal transmission with WDM signals.
In order to maintain the communication in case of a failure, such as in case of a break of a fiber connection, or in case of downtime due to maintenance, common WDM networks use protection paths which can be used alternatively for the transmission instead of working paths.
FIG. 1 shows a conventional WDM network comprising a first ROADM 10 of a first node and a second ROADM 12 of a second node. The first node and the second node are connected via a working path 14 and a protecting path 16, wherein in the WDM network identical WDM signals are transmitted via the working path 14 and the protecting path 16 from the first node to the second node.
The first ROADM 10 comprises a first wavelength selective switch (WSS) 18 for multiplexing and the second ROADM 12 comprises a second WSS 20 for de-multiplexing. The first ROADM 10 further comprises an optical splitter 22, which receives a multiplexed WDM signal provided by the first WSS 18. For simplicity only one input signal is shown for the first WSS 18. However, the WSS 18 may receive plural WDM signals at separate ports which can be multiplexed to provide a single multiplexed WDM signal. This multiplexed WDM signal is provided to the optical splitter 22. In the optical splitter 22 the incoming multiplexed WDM signal is split into two identical WDM signals carrying the same information, wherein one of these WDM signals is transmitted via the working path 14 towards the second node and the other WDM signal is transmitted via the protecting path 16 towards the second node.
The second ROADM 12 of the second node further comprises an optical switch 24 with two input ports and an output port. One of the input ports is connected to the working path 14 and the other input port is connected to the protecting path 16. The output port is connected to the second WSS 20. In case of normal operation, when no failure is present in the working path 14 and no maintenance is performed in the working path 14, the optical switch 24 only forwards the WDM signal being received from the working path 14 to the second WSS 20. The the WDM signal arriving via the protecting path 16 is not forwarded to the second WSS 20.
In case of a failure in the working path 14 the transmission via the working path 14 is interrupted and the optical switch 24 is switched to a protecting position in which the WDM signal arriving via the protecting path 16 is forwarded to the second WSS 20. When the failure is fixed and the transmission via the working path 14 resumed, the optical switch 24 can be switched back to a working position. The multiplexed WDM signal which is received at the second WSS 20 is de-multiplexed into separated WDM signals being output at separate ports of the second WSS 20. For simplicity only one of these de-multiplexed WDM signals, which are output at the second WSS 20, is shown in FIG. 1.
Both the working path 14 and the protecting path 16 can correspond to a single optical fiber or to other optical multiplex sections (OMS), which may comprise additional and/or alternative components. In the network of FIG. 1 each of the working path 14 and the protecting path 16 comprises an optical fiber, a booster 26 and a pre-amplifier 28, for example.